1. Field of the Invention
The present invention relates to an image forming apparatus that controls the rotation phases of a plurality of photosensitive members.
2. Description of the Related Art
In recent years, electrophotographic color image forming apparatuses of a tandem type have been in widespread use. In image forming apparatuses of a tandem type, toner images of individual colors are formed on a plurality of photosensitive drums, and the toner images of the individual colors are overlaid and transferred onto a recording sheet. Electrophotographic color image forming apparatuses of a tandem type have an advantage over color image forming apparatuses that form an image using a single photosensitive drum in terms of the productivity of image formation. However, color image forming apparatuses of a tandem type may cause color misregistration in which the relative positions of the toner image of individual colors are shifted when the toner images formed on the photosensitive drums are overlaid. In such a case, an unclear and low-quality image is formed on a recording sheet.
One of the reasons for causing color misregistration is a shift of rotation phase among the plurality of photosensitive drums. The eccentricity of the photosensitive drum with respect to the rotation shaft causes the peripheral speed to periodically vary. If the phases of periodical variations in the peripheral speeds of the photosensitive drums are not the same, increases or decreases in the peripheral speeds of the photosensitive drums are not the same. Therefore, shifts of the relative positions of toner image of individual colors become noticeable. To address such an issue, U.S. Pat. No. 6,173,141 describes an image forming apparatus that prevents color misregistration by performing control so that the rotation phases of the photosensitive drums are the same. In the image forming apparatus described in U.S. Pat. No. 6,173,141, a microprocessor receives the rotation speed of a photosensitive drum for color image formation and the rotation speed of a photosensitive drum for monochrome image formation and controls a drive motor for driving the photosensitive drum for color image formation, a drive motor for driving the photosensitive drum for monochrome image formation, and a drive motor for driving a transfer conveyer belt so that the variation cycles of the two drums are synchronized with each other.
A variety of techniques for phasing two photosensitive drums have been proposed. One of the techniques is to increase or decrease one of the rotation speeds of the two photosensitive drums after the rotation speeds have reached the rotation speed for image formation. If the intention is only to make two photosensitive drums be in phase, the rotation speed of either photosensitive drum can be changed. However, when the drive motor that drives the photosensitive drum for monochrome image formation also drives the transfer conveyer belt and if the two photosensitive drums are made to be in phase, the following problems arise.
That is, if the photosensitive drum for monochrome image formation is accelerated, a roller for driving the transfer conveyer belt is also accelerated. The transfer conveyer belt is stretchable. Accordingly, if the roller for driving the transfer conveyer belt is accelerated, part of the transfer conveyer belt that is pulled by the roller stretches, and part of the transfer conveyer belt that is fed by the roller is loosened. This gradually disappears due to the restoration force of the transfer conveyer belt. However, the restoration force serves as a disturbance and is applied to the roller. Therefore, it takes a relatively long time until the rotation speed of the roller reaches a predetermined stable speed through feedback control. Such a problem also arises when, in image forming apparatuses that transfer a toner image formed on a photosensitive drum onto a recording sheet via an intermediate transfer belt, the phase of the photosensitive drum is made the same as that of another photosensitive drum by increasing or decreasing the rotation speeds of drive motor for driving the photosensitive drum and the intermediate transfer belt. As described above, in order to make the two photosensitive drums be in phase, which one of the photosensitive drums is selected for increasing or decreasing the rotation speed needs to be determined.